Communication using an optical fiber has prospered in the recent years. It is very important to construct optical-fiber network systems for future communication requirements. Mainly because the use of optical signals has several advantages, such as being instantaneous, increased precision and a wider range of frequency. The demand for the optical-electronic integrated circuits is increasing.
There are two kinds of photo detectors for receiving the incident light signal in the field of communication:
(1) compound semiconductor photo detectors; and,
(2) silicon photo detectors.
The compound semiconductor photo detector is well studied in the academic research and in its application in the field of the industrial development. It is, however, more expensive than silicon photo detectors from the viewpoint of cost. Moreover, the process of making the compound semiconductor photo detector is not compatible with the integrated circuit processing. Therefore, it becomes unrealistic to produce such an integrated circuit with the capability of detecting a light signal. Thus, the cost of manufacturing this kind of integrated circuit is high and its application is also limited for the same reason.
There are three types of silicon photo detectors:
(a) APD (avalanche photo diode);
(b) pin photo diode; and,
(c) metal-semiconductor photo diode.
The first type of silicon photo detector is shown in the FIGS. 1 (a) and 1 (b). The "Si-APD" is usually used for optical communication over short distances. Its advantages are a shorter response time and a smaller light receiving area, but the shortcomings are high cost and a noisy signal. The second type of photo detector is shown in FIG. 2. The "pin photo diode" is usually used for remote controls, memory reading, and facsimile optical communications. Its cost is lower but the light receiving area is larger. The third type of photo detector is shown in the FIG. 3. The "metal-semiconductor photo diode" is usually used for detecting ultra-violet and visible light. The greatest disadvantage for this type of photo detector is that the thickness of the metal layer has to be thin and transparent to incident light.
There is always some limitation on the wavelength to be detected for all types of photo detectors being used. And the integration for the "Si-OEIC", i.e., optical-electronic integrated circuit, is quite complex and difficult. As shown in the FIG. 4 and FIG. 5, the "Si-OEIC" structure is proposed by Motohiki Yamamoto et al. and is published in the IEEE Transactions on Electron Device, Vol. 142, No. 1, pp. 58-63, 1995. From those figures, the process for the manufacturing this kind of product is quite complex and the cost is also high.
Therefore, this invention incorporates modifications from the results of studying the "Metal-Semiconductor-Metal photo diode", as applied to the optical-electronic integrated circuit, and is such that the integration of "OEIC" can be achieved. Not only can the cost of production be reduced, but also the materials for the appropriate wavelength can be selected for manufacturing an optical-electronic integrated circuit of a "metal-semiconductor-metal photo diode".